High intensity discharge (HID) lamps with translucent burners are well known in the art. The existing HID product rang consists of mercury vapor (MV), high pressure sodium (HPS), and quartz metal halide (MH) lamps. In recent years, ceramic metal halide lamps (for example, Philips MasterColor® Series) have entered the market place. Compared to the quartz MH lamps, the ceramic lamp have higher color consistency, better color rendering property, and higher lumen maintenance. The MasterColor® lamps are versatile light sources, since they can be mounted in either regular glass or quartz bulbs or in PAR reflectors. Existing Philips MasterColor® ceramic metal halide lamps include such lamps having a wattage of 39 W-150 W, also referred to as CDM lamps). Recently, the MasterColor® lamp series has been extended via work performed in our laboratory to higher wattages (up to 1000 W). These ceramic metal halide lamps display excellent initial. color consistency, superb stability over life (lumen maintenance >80%, color temperature shift >200 K at 10,000 hrs), high luminous efficacy of >90 lumens/watt, color rendering index (CRI) of >90 and lifetime of about 20,000 hours. These highly desirable characteristics are due to the high stability of the polycrystalline alumina (PCA) envelopes and a special mixture of salts, which emits a continuous-spectrum light radiation close to natural light and other parameters that have been identified and developed.
One current design of high wattage MasterColor® lamps utilizes a cylindrical PCA discharge tube with extended plugs for securing electrodes. The approximate range of aspect ratio of the PCA discharge tube, i.e. length/diameter, of the PCA body is about 3 to 10, with the distance between two electrodes ranging from 10 mm to 60 mm. For the top of the line 400 W and 1000 W lamps, the amp current is approximately 4.5 A (ANSI standard) in steady state operation and is approximately 7-8 A during warm up. The mount structure of the high wattage MasterColor® lamps include a standard glass bulb with gas filling or vacuum, stem, connectors, getters, current carrying frame wire, and ignition aids such as UV enhancer or antenna. In the current design, a single straight frame wire is mounted on the stem, and is similar to most HPS lamp constructions. The distance between the frame wire and arc tube surface is about 15 mm.
The large current carried by the frame wire generates an AC magnetic field. The magnetic field interacts with the electrons and ions of the plasma stream. We have observed that the magnetic force is strong enough to interact with the plasma stream and results in arc bending. In vertical orientation as shown in FIG. 2, the magnetic field pushes the plasma stream away from the axis of the arc tube. The maximum working temperature of the PCA surface should not exceed 1250° C. Otherwise the surface would be damaged. The curving arc, which creates a much higher local surface temperature on the opposite side of the frame wire, would result in a PCA damage or even cracks in a short period of time. In horizontal orientation, as shown in FIG. 3, the arc is naturally off-center toward the upper surface, because of the heat convection surrounding the arc tube. Placing the frame wire on the top of the arc tube would center the arc, because of the canceled forces of the heat convection and the magnetic field. However, if the frame wire were placed underneath the arc tube, the combined forces of heat convection and magnetic field would result in a more severely bent arc. With such an orientational dependence, a ceramic metal halide lamp with a straight frame wire may not be used as a universal lamp. From past experience, it is cost-prohibitive to produce special-based lamps and fixture sockets to ensure every horizontal burning lamp is mounted with the frame wire on the top of the arc tube.
U.S. Pat. No. 2,930,920, discloses an electric discharge lamp that utilizes a quartz glass discharge tube of spherical shape and of such dimensions that the lamp operates at temperatures of about 800° C. to 1000° C. In such lamp construction, the electrodes are arranged very close to each other and the arc tube is spherical and very short, i.e. the length of the arc varied from a few tenths of a millimeter to several millimeters. This translates to an aspect ratio of about 1.0. In such lamps also the electrodes are said to deteriorate under operating pressures as a result of wandering arc discharge and bowing of the discharge which is disclosed to be mitigated by various frame wire arrangements in which input leads are so connected to the frame and the frame to one of the electrodes so that the current is effectively divided to provide equal current distribution on both sides of the arc, whereby the magnetic fields associated with the sides of the frame effectively counteract each other. The compact arc, mercury vapor type lamp disclosed with a relatively short, spherical, thick wall quartz arc tube and electrodes closely spaced together, is largely obsolete by today's standard. The HID ceramic metal halide lamps of the type under consideration in this invention have excellent initial color consistency, superb stability over life (lumen maintenance >80%, color temperature shift <200 K at 10,000 hrs), high luminous efficacy of >90 lumens/watt (vs. 50-70 lumens/watt of the quartz lamps), high color rendering index (CRI) of >90 (vs. CRI<40 of the mercury vapor lamps) and a lifetime of about 20,000 hours (no deterioration of electrodes). These highly desirable characteristics are due to the high stability of the polycrystalline alumina (PCA) envelopes and a special mixture of salts, which emits a continuous-spectrum light radiation close to natural light, and other parameters that have been identified and developed in our research facilities.
U.S. Pat. No. 5,606,223 discloses a luminaire in which any noise occurring due to generation of a magnetic field is reduced through use of a structure wherein a lamp is connected at one electrode to an ordinary current loop and at the other electrode to another current loop which is branched into more than two which are extending along opposite sides of the lamp. Ceramic metal halide lamps are not disclosed.
Great Britain Patent Application 2,138,629A relates to high frequency gas discharge lamps with a gas discharge tube arranged so that at least two current-providing loops are formed whose resulting magnetic moment oppose and tend to cancel each other out, thereby reducing radio-frequency noise. Attempts in our laboratory to use high frequency ballasts to operate high wattage ceramic metal halide lamps were unsuccessful, either because they did not exhibit good lighting properties and exhibited low output, color separation, or displayed an unstable arc.
There is a need in the art for HID lamps of the ceramic metal halide type with power ranges of about 150 W to about 1000 W, and for such lamps in which the arc bending problem is eliminated or at least minimized, regardless of the orientation of the lamp in the fixture and regardless of the relative position of the frame wire to the arc tube.